Pages

Thursday, 24 January 2013

In the last post,
I reviewed Geoff's first paper looking at whether people can perceive
the affordance for throwing an object to a maximum distance and a first
swing at identifying the information specifying the affordance. People
can perceive the affordance. Bingham et al then identified an invariant
relation between the timing of the motions of the wrist and elbow when
people hefted the balls they chose as optimal for throwing, and showed
that this kinematic pattern specified a peak in the function which
determined how much kinetic energy was transferred to the ball. They
suggested that this relation in the joint movements served as
information for the dynamic property which led to a maximum distance
throw, and that this is how hefting was able to provide information
about throwing. They suggested that this was a smart perceptual mechanism for perceiving the affordance property.That was where things stood until Zhu & Bingham (2008)
ran an extensive replication and extension of the original study, to
test the specific smart perceptual mechanism proposed by Bingham et al
(1989). Experiment 1: Replicating Bingham et al, 1989This
experiment had ten people first heft then throw objects to a maximum
distance. There were 6 object sizes covering the full range of graspable
objects (1-6in). Within each size there were 8 weights. The exact
weights differed (see Zhu & Bingham, 2008, Table 1) because of
limitations in the materials used; but within each size each weight
increase was 1.55 times the previous weight. This constant geometric
progression preserved the relational structure of the weight
distribution within each size.Participants hefted
weights in increasing order within each size and ranked their top three
choices for throwing to a maximum distance. The weighted average of the
selected weights provides an estimate of the actual preferred weight for
that size, overcoming the fact that there were only a finite number of
actual options, samples from a continuous relation. In a separate
session, all the participants threw all of the objects as far as they
could three times in a random order.Participants all threw
with varying ability and picked different weights within each size,
mostly explained by variations in the size of the participant. However,
they all showed the same pattern as Bingham et al 1989; they picked
heavier weights for larger sizes, and threw their preferred objects the
farthest. To compare people, Zhu and Bingham rescaled the selected
weights as a function of the preferred weight (to get everyone on the
same scale) and plotted the distance thrown (Figure 1)

Figure 1. The distance thrown as a function of ball size and weight (scaled by preferred weight)

)

For each
size, follow the line along the rescaled weight dimension. There is a
peak distance for each combination, and that peak is projected down onto
the floor of the plot. 0 on the rescaled weight axis corresponds to the
preferred object and the line is arranged around the preferred object
weight. As in the previous study, people can heft objects and perceive
the affordance for throwing to a maximum distance.Experiment 2: What if you heft with something other than your hand?Bingham
et al 1989 suggested that people choose heavier weights as size
increases to offset the size-induced increased in stiffness about the
wrist, which would in turn preserve the timing between the elbow and
wrist movement that optimised the flow of kinetic energy along the arm
to the projectile. This, they suggested, was the smart perceptual mechanism
people were employing to perceive a throwing affordance via hefting. If
this is the case, then hefting without involving the wrist would
interfere with this perception, even though both size and weight were
being perceived.

Two
groups of participants therefore hefted the balls with either their
elbow or their feet. They could see the size and feel the weight; but
could they perceive the affordance? The answer is not really; their
judgements were much more variable and they systematically selected
objects that were too heavy

Experiment 3: Hefting with your foot to throw with your footThe
smart perceptual mechanism being investigated suggests that people can
use hefting-by-hand to perceive throwing affordances because the
dynamics (specifically, the optimal way to transmit force) of the two
tasks overlap. This isn't the case for the elbow or the foot; but can
people select objects they can throw with their foot the farthest if
they heft with their feet?

The
answer is no, not really. People's judgments were fairly random, and
people did not typically throw their preferred objects the farthest.
This presumably would get better with practice, but in general
Experiments 2 & 3 together suggest that the perception of the
throwing affordance relies on the overlap between hefting with the hand
and throwing with the hand, because that's how we typically interact
with the dynamics of projectile motion.Experiment 4: Do size and weight both matter for achieving maximum distance?The
specific smart perceptual mechanism proposed by Bingham et al (1989) is
that both size and weight affect the dynamics of both hefting and
throwing in the same way. Weight is known to affect the dynamics of
throwing; as weight increases, release velocities come down. What about
size?4 participants threw objects of 4
sizes, 8 weights within each size. The authors measured the release
angle produced (which was, on average, a quite low 24°; the optimal
angle for maximum distance given air resistance is about 36°). Zhu &
Bingham then used these angles and the thrown distances for these
objects from Experiment 1 in a simulation of projectile motion to
estimate the release velocities (this reflects limited access to the
necessary equipment, I think; our more recent work measures these
release parameters directly from the high speed video footage). Once the weights reached .05kg,
the release velocities decreased with increasing weight. However,
neither release angle nor release velocity showed any consistent effect
of size, suggesting that only weight affects the dynamics of throwing.
Size has it's effect via the dynamics of projectile motion; as size (specifically, the cross sectional area) increases,
drag increases, and distance goes down. Zhu & Bingham conclude that
the specific smart perceptual mechanism proposed in Bingham et al
(1989) is not correct.SummaryThis
paper investigated whether hefting an object provides information about
that object's throwability to a maximum distance via the smart
perceptual mechanism proposed in Bingham et al (1989). That paper
suggested that both hefting and throwing share certain dynamical
features. Throwing entails transmitting force along the arm and the
efficiency of this depends on achieving a particular timing pattern
between the limb segments. This force is stored briefly in the tendons
about the wrist and then delivered to the projectile in a fast snap.
This snap and the limb segment timing are preserved if object weight
increases as object size does, because the increase in size affects the
wrist dynamics. So Bingham et al suggested that both object size and
weight affect the dynamics of hefting and throwing identically, and
hence the former can produce information about the affordances for the
latter.Experiments 1-3 of the current
study support this hypothesis; people can perceive the throwing
affordance but only when hefting by hand. However, Experiment 4 suggests
that this specific smart mechanism might not work, because object size
does not affect the dynamics of throwing (no effect on release angle or
velocity). It does affect distance, but it does so by affecting the
dynamics of projectile motion. In order to learn to perceive throwing
affordances via hefting, therefore, people may need to have experience
seeing how far balls of different sizes travel when thrown (i.e.
experience with the dynamics of projectile motion). The next paper I'll
review examines this hypothesis in more detail.